The present invention relates to a consumable pyrogen igniter (or ignition device) for a solid rocket motor.
The purpose of an igniter of the abovementioned type is:
to ignite the solid rocket motor;
to avoid any overpressure which may be created during ignition or as a result of ignition;
to ignite the solid propellant in such a manner as to establish a uniform increase in the pressure, i.e. to avoid any pressure oscillation;
to complete the ignition in a few tenths of a second (normally within a period of time less than 100 ms or at most 200 ms).
Solid rocket motors are ignited either by a pyrotechnic igniter or by a pyrogen igniter.
Pyrotechnic igniters act in such a way as to direct onto the surface of the solid propellant of the rocket motor very hot particles which, in their turn, ignite the solid propellant. The combustion of this solid propellant has to achieve the pressurization of the rocket motor.
Pyrogen igniters contain a "gas generator". The very hot gas originating from this "generator" is intended to raise the pressure of the rocket motor. In addition to the emission of very hot gas, pyrogen igniters, just like pyrotechnic igniters, often direct very hot particles onto the surface of the solid propellant of the rocket motor in order to improve the ignition.
An essential difference between pyrotechnic and pyrogen igniters consists in that the latter contain one or more small passages which, as long as the pressure in the rocket motor is sufficiently low, act as "sonic throats", so as to decouple (at least initially) the igniter combustion process from the combustion process of the rocket motor.
Every igniter comprises the following components: an initiator (or initiator device), which can be activated electrically, but which may alternatively be activated mechanically;
a transfer device for the flame for initiating the ignition (or ignition device) which, in its simplest construction, consists of a passage permitting the hot product originating from the combustion or from the detonation of the initiator to pass along so as to reach the igniter. Frequently, this transfer device may comprise a plurality of components and, in many applications, it is possible to provide, for reasons of safety (i.e. to avoid an ignition due to human intervention errors), means for interrupting the transfer, which means consist of a safety and armament case (SAFE/ARM).
The transfer of the flame for initiating the ignition may be obtained by means of detonating cords, fast burning ignition cords, shock wave or through bulkhead initiators (TBI) or any similar pyrotechnic means capable of permitting the transfer of the flame for initiating the ignition. Moreover, the transfer device may further contain a "boost" device to ignite the igniter. Such a "boost" device may consist of a small container with a (pyrotechnic) charge, such as a ground propellant, BKNO3 pellets or any similar pyrotechnic agent. The igniter comprises a case which is fully or partially filled either with a pyrotechnic mixture (in the case of a pyrotechnic igniter) or with propellant (in some cases together with one or more pyrotechnic mixtures). The case contains one or more holes through which the combustion products enter the cavity of the rocket motor.
In the prior art, the case of an igniter is normally made of metal and, on some occasions, is integrated with the case of the rocket motor or the nozzle. In certain cases, the igniter is of the demountable type.
During rocket motor combustion, fragments of the case of the igniter are sometimes expelled through the nozzle of the rocket motor, thus damaging this nozzle or other parts of the rocket motor. To prevent this, the igniter is in most cases protected by a thermal insulation. It should be emphasized that, where the igniter is integrated with the components of the rocket motor, this may result in an increase in the mass of the system since, in the case where such an integration is performed, it is necessary to take special precautions with regard to construction, strength, heat transfer and any possible undesired interaction between the "empty" igniter--i.e. an igniter whose combustible charge has been entirely consumed, so as to leave a (resonant) cavity empty--and the rocket motor during the operation of the latter. The demountable igniter referred to hereinabove requires special means for fixing it inside the rocket motor; this is difficult in the case of modern spherical rocket motors, and frequently demands the adoption of special constructions in order to obtain a gastight closure. Measures also have to be taken to avoid any interaction between the "empty" igniter referred to above and the rocket motor during the operation of the latter.
Pyrotechnic igniters in current use comprise a metal case (sometimes thermally insulated) containing a pyrotechnic mixture. This metal case contains one or more passages through which very hot particles are directed towards the surface of the propellant to be ignited.
The pyrogen igniter is especially suitable, by reason of its characteristics, for rocket motors having a small initial free volume as well as a cylindrical geometry and a fast burning propellant, as are frequently found in military applications The pyrogen igniter is also especially suitable for mounting in the head (or head end) of the rocket motor, as the propellant ignited near the igniter may, on account of the transfer of the flame, ignite the remainder of the propellant. Most pyrogen igniters are demountable and screwed into the head end of the rocket motor.
In view of the fact that, most of the time, it is necessary to prevent the pyrogen igniters from being ejected through the nozzle, they are of a heavy construction, which consequently gives rise to an increase in the mass of the system as a whole. Pyrogen igniters may be integrated in the submerged part (i.e. in the part which projects into the cavity of the rocket motor) of the nozzle (as, for example, in the case of the Italian satellite IRIS), but may also be mounted in the head end of the rocket motor (as, for example, in the case of the European geostationary apogee motor, MAGE, which is a large cylindrical booster apogee motor developed for European satellites). In each case, a special layout has to be provided to mount or integrate the igniter and it is necessary to take precautions to ensure that no unacceptable "hot spot" occurs, the adoption of these precautions again leading to an increase in the mass of the system. If the pyrogen igniter (and in some cases also the pyrotechnic igniter) is expelled through the nozzle, after it has ignited the rocket motor, special precautions also have to be taken to avoid the temporary blockage of the nozzle and/or damage to this nozzle. Furthermore, other precautions have to be taken to ensure that the metal parts do not damage the combustion surface of the solid propellant.
The pyrogen and pyrotechnic igniters of the type integrated with components of the rocket motor or mounted separately in the rocket motor are systematically used in rocket motors intended for military and space applications, although for space applications and for modern rocket motors there is a strong preference for pyrogen igniters.
The typical arrangement of a pyrogen igniter in a modern solid rocket motor for space craft propulsion is illustrated in FIG. 1, in which the following reference numerals designate the components indicated:
1, the solid propellant of the rocket motor;
2, the polar fitting of the ignition device;
3, the igniter, which is demountable and mounted on the head end of the rocket motor;
4, the shock wave or through bulkhead initiator;
5, the rigid explosive transfer line;
6, the flexible explosive transfer line.
The layout of a typical metal pyrogen igniter is illustrated in FIG. 2, in which the following reference numerals designate the components indicated:
7, the thermal protection;
8, the metal case of the combustible charge of the igniter;
9, the pyrogen combustible charge;
10, the threaded ring;
11, the front cover of the combustible charge;
12, the seal;
13, the pressurization valve;
14, the thermal protection of the cover;
15, the cover;
16 the shock wave or through bulkhead initiator;
17, the rigid explosive transfer line;
18, the end cover of the combustible charge;
19, the nozzle inlet.
A consumable pyrogen igniter is described in the U.S. Pat. No. 3,392,673 (KING). This igniter, which is intended for a rocket motor having a solid propellant with a longitudinal central perforation, comprises:
a) a tubular case, comprising a thin layer of glass cloth impregnated with resin and a relatively thick external reinforcing layer made of a combustible material impregnated with an oxidizing agent, the latter permitting the combustion of the tubular external reinforcing layer during the combustion of the abovementioned solid propellant;
b) a combustible charge in the form of a layer lining the internal surface of the reinforced tubular case referred to hereinabove;
c) a combustion chamber delimited by the case lined in this manner;
d) an initiator for ignition of the igniter;
e) a nozzle, the diameter of which is chosen in such a manner as to maintain the desired pressure within the combustion chamber of the igniter, this nozzle being made of a material which is heat-resistant and non-consumable.
However, the ignition of the case containing the combustible charge of the igniter takes place only after the igniter has performed its function, so that the combustion of this case does not contribute to the useful thermal power of the igniter, thereby reducing the energy output and at the same time contributing to the increasing of the volume and of the weight of the igniter. Furthermore, many components are made of gas-generating organic materials, some of which are not consumable, so that this igniter has relatively large dimensions.